1. Field of the Invention
The present invention is related to a filter having a constant input impedance over frequency.
2. Background Art
In addition to data communications, the Internet can also be used to carry voice telephony. One conventional system that carries voice communications over the Internet utilizes an Internet Protocol (IP), and such telephones are referred to as IP telephones.
The data terminal equipment (DTE) of an IP telephone includes a telephone line that is connected to a computer device through a series-connected relay (i.e. switch). The relay switches an incoming telephone signal to either the computer or to a filter that is connected in parallel with the computer. The filter is connected/disconnected across the computer depending on the state of the IP phone system by closing/opening the associated relay. In a no power or xe2x80x9cdiscoveryxe2x80x9d mode, the relay is switched so the filter is connected across a physical layer input of the computer. Therefore, the filter receives an incoming signal on the telephone line and passes low frequency signals back down the telephone line, without the incoming signal reaching the physical layer of the computer. The reflected low frequency signals indicate that a compatible IP phone is available for use. When power is applied to the relay in a xe2x80x9cnormal operationxe2x80x9d mode, the relay is switched so the filter is disconnected from the input of the physical layer of the computer. Therefore, the filter does not effect the incoming signal, and the incoming signal is applied to the physical layer of the computer for further processing.
The continual opening and closing of the relay creates wear and tear on the relay components as the conventional IP phone switches between the discovery and normal modes, eventually causing component failure. It would be more cost-effective to keep the filter connected at all times, thereby eliminating relay replacement. Additionally, the conventional relay is not integrated with the computer or the filter, which increases the manufacturing part count and ultimately the manufacturing cost of an IP Phone.
The filter in the conventional IP telephone is a conventional lowpass filter.Conventional lowpass filters have an input impedance that is highly dependent on the frequency of the input signal that is delivered to the filter. In other words, the input impedance varies with the frequency of the input signal. Input frequencies that are inside the passband of the filter see a good impedance match and are substantially passed to the filter output. Input frequencies that are outside of the filter passband are substantially reflected, which causes an undesired high return loss that can reduce sensitivity in the IP phone system.
Additionally, conventional filters are highly sensitive to variations in the filter components and in the variation of components that are connected to the filter. This is undesirable as small changes in the filter components can cause large variations in the electrical characteristics of the filter.
What is needed is a filter that has a constant impedance for all frequencies, even frequencies that are outside the passband of the filter. Furthermore, the filter should be relatively insensitive to component variation.
The present invention is a constant impedance filter that maintains a constant input impedance through the filter for frequencies that are both inside the filter passband and outside the filter passband. The constant input impedance appears as a pure resistance to the incoming signals. In other words, frequencies both inside and outside the filter passband see a substantially matched impedance. Frequencies that are inside the passband are passed to the filter output. Frequencies that are outside the passband are terminated inside the filter, and are not reflected.
The constant impedance filter includes a plurality of filter poles that are connected in series. Each of the filter poles include an inductor, a capacitor, and a resistor. The value of the inductor, the capacitor, and the resistor are selected to provide a constant input impedance over frequency for each pole of the filter, which produces a constant input impedance for the entire filter over frequency. In embodiments of the invention, the filter components for each filter pole adhere to the equation, C=L/R2, producing a constant input impedance of R for each filter pole. An entire filter will have a constant input impedance if the equation C=L/R2 is maintained for each individual filter pole.
The constant impedance filter can have several embodiments depending on the type of frequency signals that are being processed. The filter embodiments include a lowpass filter, a highpass filter, and a bandpass filter, all having a constant input impedance over frequency. Furthermore, the lowpass, highpass, and bandpass filter embodiments can be constructed in both single-ended and differential circuit configurations.